Triarylamine-Substituted Benzimidazoliums as Electron Donor-Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations.

Triarylamine-substituted benzimidazoliums (BI + -PhNAr 2 ), new electron donor-acceptor dyad molecules, were synthesized. Their photocatalytic properties for reductive organic transformations were explored using absorption and fluorescence spectroscopy, redox potential determinations, density functional theory calculations, transient absorption spectroscopy, and reduction reactions of selected substrates. The results show that irradiation of BI + -PhNAr 2 promotes photoinduced intramolecular electron transfer to form a long-lived (∼300 μs) charge shifted state (BI • -PhN •+ Ar 2 ). In the pathway for photocatalysis of reduction reactions of substrates, BI • -PhN •+ Ar 2 is subsequently transformed to the neutral benzimidazolyl radical (BI • -PhNAr 2 ) by single-electron transfer from the donor 1,3-dimethyl-2-phenylbenzimidazoline (BIH-Ph) serving as a cooperative agent. Among the benzimidazoliums explored, the bromo-substituted analogue BI + -PhN(C 6 H 4 Br- p ) 2 in conjunction with BIH-Ph demonstrates the most consistent catalytic performance.